The present invention relates generally to airfoil blades for use in turbo machinery and, more specifically, to a turbine blade having improved friction damping characteristics.
The advantages of providing turbine blade damping are well known. During engine operation, turbine blades are exposed to a wide variety of stresses due to heat, oscillating airflow, etc., and correspondingly exhibit many adverse vibration modes and manners of operation. As a consequence, large stresses are induced in the blades and other elements of the turbine engine. These dynamic stresses can cause fatigue failures of the blade material and other components and, can even cause catastrophic failures of the engine and perhaps the aircraft itself. Thus, damping is introduced to offset these adverse consequences in order to reduce or even eliminate these stresses.
Various attempts have been made to date to provide damping to turbine engine internal components. For example, U.S. Pat. No. 5,522,705 to Elaini et al. discloses a friction damper for gas turbine engine blades utilizing a plate mounted underneath the outer shrouds of two adjacent airfoils. The friction generated between the plate and the shroud provides a degree of damping to reduce vibrations in the turbine blades. A disadvantage of systems like these is the increase of undesirable rotating mass in the turbine rotor as well as increased complexity of assembly of the engine.
Another attempt at turbine blade damping is found, for example, in U.S. Pat. No. 5,369,882 to Dietz et al. The Dietz apparatus includes a damping mechanism retained within the platform section of the turbine blade. The damping mechanism includes a damping wedge slidably received within a machined recess within the platform of the turbine blade. During operation, the damping wedge slides within the machined recess, frictionally engaging the walls of the recess, providing a degree of damping.
While damping systems such as these are somewhat effective, a limitation lies in the fact that the blade cannot be heat treated after final welding and correspondingly cannot be effectively stress relieved. Moreover, the damping action depends on the movement of the damping device which is dependent on turbine rotor speed. As is known, the undesirable stresses of operation can occur in very narrow ranges of operation and can even occur at different, perhaps unexpected, engine speeds based on transient operating conditions both internal as well as external to the engine.
U.S. Pat. No. 3,758,233 to Cross et al. is an example of the technique of covering the external surface of the turbine blade with a coating intended to provide damping to the turbine blade. A disadvantage to this technique is that the coating is not sufficiently durable for implementation into modern turbine engines because the coating has been found to craze or even crack off after several thermal cycles.
The problem of providing efficient turbine blade damping becomes compounded because as engine designs progress, the implementation of new low aspect ratio turbine blade designs for use either singly or within one-piece, manufactured blade and disk combinations called xe2x80x9cblisksxe2x80x9d has become more and more widespread. These low aspect ratio turbine blades enhance engine operation but have the disadvantage of exhibiting more complex modes of vibration and, they are thinner, rendering it more difficult to provide efficient damping and also permitting less physical room to incorporate damping mechanisms.
A need exists therefore for an improved friction damped turbine blade. Such a blade would exhibit improved friction damping during turbine operation, enhancing engine and blade performance as well as contributing to increased longevity.
Accordingly, it is a primary object of the present invention to provide a friction damped turbine blade overcoming the limitations and disadvantages of the prior art.
Another object of the present invention is to provide a friction damped turbine blade exhibiting improved friction damping during turbine operation, enhancing performance as well as blade longevity.
Another object of the present invention is to provide a friction damped turbine blade having a body formed of segments, each segment being in slidable contact with an adjacent segment for providing effective frictional damping.
Yet another object of the present invention to provide a friction damped turbine blade having a body formed of interlocking segments in slidable contact, for providing effective frictional damping while maintaining a continuous airfoil surface during turbine operation.
Still another object of the present invention is to provide a friction damped turbine blade having a body formed of slidably engaged segments including a friction reducing powder interposed between the segments for enhanced damping effect while reducing wear.
Additional objects, advantages and other novel features of the invention will be set forth, in part, in the description that follows and will, in part, become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects and in accordance with the purposes of the present invention as described herein, a friction damped turbine blade incorporates a body formed of at least two segments in slidable contact with each adjacent segment. During turbine operation, the segments slidably engage one another, providing enhanced turbine blade damping, thereby enhancing turbine engine operation as well as enhancing longevity.
According to an important aspect of the present invention, the sliding action of the adjacent segments provides frictional damping to the turbine blade. Depending on turbine blade configuration, mass distribution requirements, etc., several segments can be utilized to form the body of the turbine blade. Advantageously, this method of providing frictional damping introduces damping into the blade while avoiding the introduction of large stress concentrations in the blade, enhancing blade longevity as well as turbine engine reliability.